For the assessment of children's asthma status we used the Greek version of the ISAAC (International Study of Asthma and Allergies in Childhood) core questionnaire supplemented with questions relating to potential person-based risk factors such as the place of birth of the child, ethnic origin of the parents, birth order, parental education level, parental smoking, history of atopy in the immediate family (siblings and parents with a history of asthma, eczema and or hay fever), personal uptake of smoking, pet ownership and place of residence.